Rechargeable electrochemical generators manufactured from thin film laminates of electrolytes and sheet-like anodes and cathodes display many advantages over conventional electrochemical generators. These advantages include lower overall battery weight, high power density, high specific energy, and longer service life.
Components of a lithium polymer electrochemical cell include a positive electrode, a negative electrode, and a separator capable of permitting ionic conductivity such as a solid polymer electrolyte sandwiched between the negative electrode and the positive electrode. The negative electrode, or anode, and the positive electrode, or cathode, are made of material capable of reversible insertion of alkali metal ions. The polymer electrolyte separator electrically isolates the anode from the cathode to prevent short-circuits between the anode and the cathode which would render the electrochemical cell useless.
The cathode is typically formed of a mixture of active material capable of occluding and releasing lithium such as transitional metal oxides or phosphates, an electronically conductive filler, usually carbon or graphite or combinations thereof, and an ionically conductive polymer binder. Cathode materials are usually paste-like materials and require a current collector, which may be a thin sheet of electrically conductive material such as an aluminum foil or an electrically conductive grid. The anode is typically made of light-weight metal foils, such as alkali metals and alloys typically lithium metal, lithium oxide, lithium-aluminum alloys and the like, but may also be made of composite paste-like material comprising, for example, carbon based intercalation compounds in a polymer binder, in which case the anode also requires a current collector support such as a copper foil or grid. Composite cathode thin films are usually obtained by solvent coating onto a current collector or by melt extrusion. Similarly, the polymer electrolyte separator layer is typically produced by solvent coating or by melt extrusion. The thin film components are often manufactured in continuous lengths (L>1000 meters) of fixed width and thereafter cut to specific lengths for assembly.
A lithium polymer electrochemical cell is manufactured by successive layering of the positive electrode, the electrolyte separator, and the negative electrode. The positive electrode material is initially coated or extruded onto a metallic foil (for example aluminum) or on a metallized plastic film which serves as a current collector. The polymer electrolyte separator is thereafter directly coated or extruded onto the previously-coated cathode material or may be laminated thereon after having been formed into a thin film. The negative electrode is finally laminated onto the electrolyte separator to form an electrochemical cell. To increase the energy density of an electrochemical cell, a bi-face configuration is often used, wherein positive electrode material is laminated, coated, or extruded onto both sides of the current collector and thereafter an electrolyte separator and a negative electrode are laminated onto each positive electrode layer to form a bi-face electrochemical cell. Electrochemical cells as previously described are thereafter stacked or wound into an electrochemical cells assembly having a specific number of cells.
For the electrochemical cells assembly to perform well and have the required life expectancy, the thickness of the thin film components which make up the electrochemical cells must be as uniform and consistent as possible. To ensure this uniformity and consistency, the thickness of the thin film components should be continuously measured as they are produced in order to maintain the thickness of the thin film within its tolerance requirements. Thin film components for electrochemical cells are extremely thin. For example, the thickness of the electrolyte separator may range from 10 μm to 30 μm with a tolerance of ±3 μm, protective layers on an aluminum foil current collector may be as thin as 2.5 μm ±1 μm, whereas the thickness of cathode films may range from 40 μm to 100 μm ±4 μm. Because of these extremely small thicknesses and the high precision required by tight tolerances, it is difficult to reliably measure the thickness of these thin film components on a continuous basis to ensure quality of the end products. The difficulties are compounded by the fact that the thin film components have variations in color, transparencies and surface roughness and therefore optical reflection techniques are not adapted for these specific films.
Some existing systems are able to precisely measure the thickness of samples of thin film held stationary using a variety of methods such as spectroscopy, electron beam, interferometry, wavelength transmitted through or reflected from a thin film, spectral imaging ellipsometry, X-rays, material density measurements with calculated extrapolation of thickness, etc. However, these systems by themselves are unable to achieve high precision when required to measure the thickness or thickness profile of a moving thin film in a production environment. Some existing systems can measure the thickness of a moving thin film in a production environment but are plagued with limited accuracy due to the constrains of the production environment. Thickness measurement systems have been devised to control the average or mean thickness of a thin film being produced but these are obviously unable to provide an accurate portrait of the thin film being produced and are unable to attain sub-micron precision.
There is thus a need for a method and an apparatus for accurately measuring the thickness of thin film components on a continuous basis in order to monitor the quality of such films in the production environment of electrochemical cell components manufacturing and assembly of electrochemical generators.